Modules of optically pumped solid-state slab lasers usually comprise sources for optical pumping and a gain element in a form of an elongated slab made in a plane-parallel form with two opposite lateral faces for direction of an output laser beam so that it propagates along the slab length in a zigzag path, undergoing total internal reflections at the two lateral faces. Usually radiation from the sources for optical pumping is introduced into the laser slab through the same lateral faces. Since the operation of high power solid-state lasers needs significant heat removal from the laser slab, cooling of the slab in such devices can be realized by liquid cooling of the same lateral faces through which pump radiation is applied. In such modules both lasers diodes and pump lamps are used as pump sources.
If in a laser module the same lateral faces through which pump radiation is applied are used for cooling of the laser slab then in the path of pump radiation there are found to be optical windows, flows of cooling fluids, antireflection coatings on window surfaces and protective coatings of the laser slab, which all result in inevitable energy losses and thereby in a decrease of pump efficiency and laser output power. Further, high requirements to optical, thermomechanical, thermophysical and beam parameters exist for the materials to be used for the optical windows and coatings in such devices. These requirements essentially limit the possibility of choice of materials and techniques and complicates the fabrication of a laser module.
These problems are addressed by laser modules with different tasks of the lateral faces of the laser slab, where heat is removed from the same pair of the opposite lateral faces which are used for waveguide propagation of a laser beam and where optical pumping is realized through another pair of opposite lateral faces. This technical solution strongly reduces the number of requirements to parameters of cooling media and coatings, and allows to use of conduction cooling of the laser slab in order to miniaturize of the entire device. Thus, an optically pumped laser module as described in U.S. Pat. No. 5,949,805 comprises the sources for optical pumping and a solid-state laser slab having a pair of first opposite lateral faces which are adapted to supply pump radiation from the pumping sources through them into the laser slab and which are spaced at a distance defining a width of the laser slab and a pair of second opposite lateral faces which are plane-parallel for direction of an outputted laser beam so that it propagates along the slab length in a zigzag path while undergoing total internal reflections at the second lateral faces, and which second lateral faces are spaced at a distance defining a thickness of the laser slab and with the slab further having a pair of end faces with the distance between them defining the length of the laser slab. This module also comprises heat-removing portion, being in thermal contact with each of the second lateral faces in order to remove heat from the laser slab. The thermal contact between the laser slab and the surfaces of heat-removing portion is achieved in U.S. Pat. No. 5,949,805 by means of a two-layer coating including a transparent layer of silicon dioxide with a thickness of 2 μm at the surface of the laser slab and a following adhesive layer with a thickness of 2-6 μm. The first layer guarantees waveguide propagation of the beam with small losses and further protects the surface of the laser slab from harmful influences of the second layer material. The second layer guarantees attachment of the slab unit to the means for removing heat.
Absorption of pump radiation by the laser slab causes heating of the slab. In case of a uniform illumination of the laser slab volume by pump radiation, of a uniform heat removal from the corresponding pair of opposite lateral faces and of absence of fringe heat effects (here and hereafter fringe heat effects shall refer to effects being caused by the opposite lateral faces of a laser slab having at least partially no contact with the heat removing portion), the temperature distribution along the thickness of the laser slab has a parabolic form with a temperature maximum in the middle of the laser slab. Due to dependence of the refractive index of the laser slab material from temperature, the distribution of the refractive index along the slab thickness obtains a similar form close to the parabolic one.
However, the laser module described in U.S. Pat. No. 5,949,805 uses a laser slab with a cross-section close to square (that is, width and thickness of the laser slab have almost the same dimensions). Since the width of fringe area of a laser slab where the above mentioned fringe heat effects take place is in first approximation the same order as the slab thickness in the well-known laser modules the area of fringe heat effect occupies a significant part of the cross-section of the laser slab. A distribution of the refractive index, which appears as a result of fringe effects, having remarkable components of higher orders than the second one, has an impact on the output laser beam which hardly can be corrected by means of sufficiently simple external optical systems. As a result, the laser beam generated by the known module has an increased divergence.
Pump power in the known laser module is delivered to the laser slab perpendicular to the first lateral faces in light beams emitted from the pump sources, which are located directly opposite the lateral faces. This pump radiation must be sufficiently uniformly distributed over the volume of the laser slab in order to increase pump efficiency and to prevent local heat overloads, which result in decreased maximum dissipated power, as well as in increased output laser beam divergence due to laser slab surface deformations. However the light beams shaped by known pump sources are characterized by a significant spatial non-uniformity of radiation intensity distribution. So, to provide uniformity of pump radiation distribution over the volume of the laser slab, the surfaces of the lateral faces through which pump radiation passes, are roughened in known laser modules. However such rough light-diffusing surface reflects (back scatters) an appreciable part of pump radiation. This negatively affects pump efficiency. Further, irregularities at the surface of the lateral faces can cause a laser slab damage when tension forces which appear in consequence of temperature variations inside the laser slab in operation arise. This limits maximum power, dissipated into the laser slab, and hence limits the laser output power.
Additionally, when irradiating the surfaces of the lateral faces with pump light in transverse direction a large part of pump energy after having transversed the lateral faces will continue to propagate in the same direction or close to it in spite of pumping light diffusion at the rough surface. A certain fraction of this energy, which is not absorbed by doping material, passes through the laser slab and is irreversibly lost. This further reduces efficiency of the pumping, particularly for a laser slab with a dopant of low concentration and small stimulated emission cross-section σ. To increase the lateral faces roughness allows some reduction of such losses, due to stronger diffusion of the pump light into a wider angle range that lengthens the pump light in the laser slab medium, however simultaneously results in increased pump energy losses due to reflections at the rough surfaces of the lateral faces.